The combination of urethane polymer and coupling agent yields a flexible film having a good range of mechanical properties and very good water vapour barrier action.
Flexible films having good mechanical properties are required for the production of flexible containers for gaseous or liquid media, such as for example water beds. It is known that plasticised PVC may be used for such applications. Due to the particular properties of PVC, relatively large quantities of material must be used in such cases.
Monolayer films of thermoplastic polyurethane (TPU), processes for the production thereof and use thereof are known from the prior art, for example from EP 0 308 683, EP 0 526 858, EP 0 571 868 or EP 0 603 680. The structures described in these documents may be incorporated as the higher-melting layer or layers in laminating films or have already been incorporated into laminating films of a known type. The production of TPU films using substantially incompatible polymers as flatting agents in TPU elastomers has also been described, for example in DE 41 26 499.
It is furthermore known that a considerable improvement in the adhesion properties of a homo-, co- or terpolymer may be achieved by adding a low molecular weight resin of a non-waxy nature, whether of natural or synthetic origin. Such resins are described, for example, in Ullmanns Encyklopadie der technischen Chemie, volume 12, 4.sup.th edition, Verlag Chemie, Weinheim 1976, pp. 525-555. Such resins conventionally have an average molecular weight of below 2000 g/mol. The resins, both those which are solid and those which are liquid at room temperature, are not usually crystallisable and accordingly do not have a well defined melting point. The materials taken to be included among low molecular weight resins do, however, have a softening temperature which may be determined using the ball and ring method according to ASTM E-28 or DIN 1995. These resins may reduce melt viscosity, so possibly rendering melt homogeneity distinctly more uniform. Due to the improvement in flow properties brought about by addition of the low molecular weight resins, films having outer layers of already flexible co- or terpolymers exhibit the disadvantage of having very smooth surfaces, so giving the films a tendency to block.
The object thus arose of providing a flexible film having a good range of mechanical properties, superior to that of plasticised PVC, combined with elevated impermeability to water vapour. In order to minimise production costs, it should be possible to achieve the combination of properties in a single stage process.